The experimental results measured Z11 and Z22 at 50MHz to 300MHz with 50MHz step, respectively. The measurement results are shown in the following graph. Red point on graph is infinite average of measure value.
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Fig. 2-11. Z11 of glucose-DI water solution (50mg/dl concentration) with -10dBm power input
Fig. 2-12. Z11 of glucose-DI water solution (300mg/dl concentration) with -10dBm power input
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Fig. 2-13. Z11 change from glucose concentration 50mg/dl to 300mg/dl at 50MHz with -10dBm power input
Fig. 2-14. Z22 change from glucose concentration 50mg/dl to 300mg/dl at 50MHz with -10dBm power input
0 50 100 150 200 250 300 350 274
276 278 280 282
Z11 (Ohms)
Glucose concentration (mg/dl)
Z11
50 100 150 200 250 300
391.0 391.5 392.0 392.5 393.0 393.5
Z22 (Ohms)
Glucose Concentration (mg/dl)
Z22
22
Fig. 2-15. Measured S21 with solution on the sensor
When the measurement frequency is 50MHz, the proposed sensor has sensitivity of 0.0208Ω.
Experimental results of inputting -10dBm to the sensor showed that the lower the frequency, the higher the sensitivity, and the lower the accuracy of the short electrode with the smaller sensing range.
At 50MHz, S21 decreased -90dB. But not like simulation result, there are multiple resonance points.
Multiple resonance points are occurred by SMA connector which is attached to the sensor for signal input.
4 Conclusions and future work
In conclusion, this study has made following contributions to non-invasive glucose monitoring sensor using impedance spectroscopy. The study was conducted using glucose and DI-water solutions for changes in blood glucose concentration.
The existing multi-electrode sensor was not modeled as the RLC circuit. This study was conducted to investigate how the new capacitance parameters represent the real-world problems.
The experimental results supporting these simulation results have improved the accuracy by reducing interference between sensor electrodes and influence of noise. Different concentrations of glucose solution were measured with the sensor. As the glucose concentration increased, the measured impedance increased. Proposed sensor has shown promising results which agrees to simulation results.
0 50 100 150 200 250 300 350 -100 -95
-90 -85 -80 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20
S21 (dB)
freq (MHz)
Measured
23
For the future work, further experiments need to be carried out by attaching the sensor to the human body.
Research on sensor read-out IC, not VNA, is required for portable blood glucose measurement devices.
In addition, the study of the blood’s dielectric properties, skin and underlying tissue at the measuring frequency is necessary for more accurate blood glucose measurement.
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